Neovascularization of the retina and other ocular tissues is associated with retrolental fibroplasia, diabetic retinopathy and other ocular pathologies. The causes of such neovascularization are unknown. Understanding the initiating events which control retinal capillary endothelial proliferation and neovascularization might facilitate understanding of the conditions and factors promoting diabetic and other retinopathies. Several growth factors have been isolated from the retina and other ocular tissues and have been found to increase the proliferation of vascular endothelial cells and/or neovascularization. Several putative inhibitors which block endothelial cell proliferation and/or neovascularization also have been identified and isolated from the vitreous humor of the eye. The vitreous derived inhibitor which has been described by our laboratory has a low molecular weight ( less than 3000 daltons) and is heat stable. It appears to contain carbohydrate moieties, but no detectable protein. Further isolation and characterization of this inhibitor is proposed. Experiments are proposed to evaluate the capacity of this inhibitor to block neovascularization in vivo and in vitro. The tissue source of this inhibitor (retina, hyalocytes, etc.) will also be investigated.